I'm in the same position as you Kev and according to my handbook it is o.k to use the same wheelnuts but they need to be torqued to a different setting. Can't remember it off hand but if someone doesn't come along with the figures I'll check out my hand book later ( the figure of 85 for a steel wheel rings a bell but not 100% on that)
You can use alloy standard wheel nuts on a steel wheel, but not the reverse as the grade of steel used in steel wheel bolts is less. Alloy wheel bolts usually have the grade 10.9 stamped on the head. The tightening torque can vary for each alloy wheel maker so check the manufacturer, a nominal torque is 85 lb/ft, however steel wheels are usually tightened to 65 lb/ft
Just be careful as John's comment is mentioning only one manufacturer. ALKO still state that they only supply steel wheel bolts for their chassis, and any alloys will need different bolts, not supplied by them.